The present invention relates to the field of lubricants. More particularly, the present invention relates to a lubricant composition with a high molybdenum content and comprising a combination of at least two compounds comprising molybdenum of a different chemical type. The lubricant composition according to the invention simultaneously has good fuel economy properties and good stability properties on storage. The present invention also relates to a method for the lubrication of a mechanical part. The present invention relates to a method for reducing energy losses by friction of a mechanical part. The use of a lubricant composition for reducing fuel consumption is also another subject of the present invention.
The worldwide spread of the automobile since the end of the last century poses problems regarding global warming, pollution, the security and use of natural resources, in particular to the depletion of oil reserves. Following the establishment of the Kyoto protocol, new standards protecting the environment required the automobile industry to construct vehicles having reduced pollutant emissions and fuel consumption. As a result, the engines of these vehicles are subject to increasingly strict technical restrictions: in particular they run faster, at increasingly high temperatures and must consume increasingly less fuel. The nature of engine lubricants for automobiles has an influence on the emission of pollutants and on fuel consumption. Engine lubricants for automobile engines, called energy-saving or “fuel-eco”, have been developed in order to meet these new requirements.
The improvement in the energy performance of lubricant compositions can be obtained in particular by mixing specific additives such as friction modifiers, polymers that improve the viscosity index into base oils. Among the friction modifiers, the organometallic compounds comprising molybdenum are commonly used. A sufficient quantity of molybdenum must be present for a lubricant composition to have good anti-friction properties. Among these organometallic compounds, molybdenum dialkylthiocarbamate (referred to in the rest of the present application by the acronym Mo-DTC) is most often used as a source of molybdenum. However, this compound has the drawback of causing the formation of sediments when the lubricant composition has a too high content of elemental molybdenum. The poor solubility of this compound modifies, or even degrades the properties of the lubricant composition, in particular its viscosity. Now, a composition which is too viscous or not viscous enough militates against the movement of the mobile parts, easy starting of an engine, the protection of an engine when it has reached its operating temperature, and therefore ultimately causes in particular an increase in fuel consumption.
Different attempts at solubilizing Mo-DTC in lubricant compositions have been tested. The use of asymmetrical Mo-DTC compounds, i.e. obtained from dialkylamines having hydrocarbon-containing groups of different sizes is known from the document EP0719851. These asymmetrical compounds, in particular in combination with Mo-DTP compounds, make it possible to improve the solubility of the molybdenum, in particular originating from the Mo-DTC, in lubricant compositions having a high viscosity index (VI).
The document EP 0 757 093 describes lubricant compositions which can comprise Mo-DTC and/or Mo-DTP. However, this document teaches that a quantity of molybdenum which exceeds 700 ppm provided by the Mo-DTC and the Mo-DTP can result in the appearance of problems of stability of the composition, thus leading a person skilled in the art away from formulating lubricant compositions with a high molybdenum content.
As fuel economy demands are increasing, a need still exists to formulate a lubricant composition having a high molybdenum content and simultaneously having storage stability and improved fuel economy properties. By “high molybdenum content” is meant within the meaning of the present invention, lubricant compositions having a total mass of molybdenum of at least 1000 ppm (ppm=parts per million) with respect to the total mass of the lubricant composition.
An objective of the present invention is to supply a lubricant composition overcoming all or part of the aforementioned drawbacks. Another objective of the invention is to supply a lubricant composition the formulation of which is easy to implement. Another objective of the present invention is to supply a lubrication method allowing energy savings. Thus, a subject of the invention is a lubricant composition comprising at least one base oil, at least one molybdenum dithiocarbamate (Mo-DTC) compound, at least one molybdenum dithiophosphate (Mo-DTP) compound and in which the quantity of molybdenum provided by the Mo-DTP compound and the Mo-DTC compound ranges from 1000 to 2500 ppm by mass with respect to the total mass of the lubricant composition and in which the quantity of molybdenum provided by Mo-DTC compound is strictly less than 900 ppm by mass with respect to the total mass of the lubricant composition.
By “lubricant composition” within the meaning of the present invention, is meant a lubricant composition, and not a grease. In fact in greases, the additives are not solubilized but dispersed in the network of fibres formed by the soap. The problem of solubility of the Mo-DTC does not arise as it does in particular in motor oils, in which solubility is essential. Thus, the lubricant compositions according to the invention are not greases.
Surprisingly, the applicant company has observed that, in a lubricant composition having a molybdenum content which ranges from 1000 to 2500 ppm and comprising an Mo-DTC compound, the addition of at least one Mo-DTP compound makes it possible to solubilize the Mo-DTC compound and simultaneously makes it possible to improve the fuel saving properties of said composition. However, the quantity of Mo provided by the Mo-DTC compound must be strictly less than 900 ppm in the lubricant composition with respect to the total mass of the lubricant composition. Thus, the present invention makes it possible to formulate lubricant compositions with a high molybdenum content and in which the Mo-DTC compounds are soluble, i.e. they can be dissolved in the lubricant composition without forming a precipitate or without making it cloudy.
Advantageously, the Mo-DTC compounds are soluble in a lubricant composition the temperature of which varies from 0° C. to 200° C., preferably from 10° C. to 150° C., more preferentially from 20° C. to 100° C., yet more preferentially from 40° C. to 80° C. Advantageously, the lubricant compositions according to the invention have a better storage stability, in particular for storage at a temperature of 0° C. Advantageously, the combination of at least one Mo-DTC compound and at least one Mo-DTP compound in a lubricant composition comprising a high molybdenum content allows fuel savings to be made when an engine is idling or running at high speed. In an embodiment, the lubricant composition essentially consists of at least one base oil, at least one Mo-DTC compound, at least one Mo-DTP compound and in which the quantity of molybdenum provided by the Mo-DTP compound and the Mo-DTC compound ranges from 1000 to 2500 ppm by mass with respect to the total mass of the lubricant composition and in which the quantity of molybdenum provided by the Mo-DTC compound is strictly less than 900 ppm by mass with respect to the total mass of the lubricant composition.